Onan Marquis 6500 Propane generator. We had quite a bit of trouble with it last year, involving several rounds of repairs. By the time we put the coach into storage last fall, we felt the generator problems were finally behind us because it was running like an absolute champ.

Today I took the coach out of storage. The generator started OK, but it was lazy. It took about ten minutes to finally get up to speed, then after about ten minutes of charging, the engine began missing and slowing down. I shut it down before it died.

My troubleshooting efforts today involved changing spark plugs, testing spark plug wires. Initially, the engine would start, but behave as if it was missing. Now, the situation has deteriorated. The engine now will fire, but not start. As long as I hold the starter button, the engine will continue to fire and kind of roll along with the starter. As soon as I release the starter button, the engine simply dies. Tomorrow I thought I'd test coil and ignition. The propane tank is more than 1/2 full, and I do smell propane a little while trying to start the engine.

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We had a similar problem many years ago on a 4K Onan gas generator. Turned out to be the condenser. I changed both the points and the condenser, and all was well. The points and condenser are located on the top center of the generator on the gas unit if I remember correctly. The other problem I am aware of with Propane generators is the fuel regulator gets clogged with oil from the Propane. I have been told you can remove the regultor and clean it out, but I have never doen that. Good luck.

After starting, when the starter is released, the control board needs voltage from the generator to operate and it needs the oil pressure switch to close the "run relay". Or the generator will stop. (low oil level?) Hard start or no start can be the fuel solenoid sticking. You can check the solenoid by applying 12 volts and listen for a click.
Besides the regulator gumming up, There is a fuel filter next to the solenoid that can be cleaned. Probably dealer items, The propane in that area is at full tank pressure and has to be purged before it is safe to open the lines. A lot of propane can escape.
Troubleshooting:http://www.flightsystems.com/pdf/onanrvtshootgd608.pdf

Onan has a service manual for the 6500 generators, as I remember it's reasonable.
They recommend starting the generator monthly and running under load for and hour or 2. Keeps everything fresh and dries moisture in the generator windings. (probably hard to do in storage)
Hope this helps

first check the tube under the regulator that it is not blocked. Had a mud dobber plug mine in one day. Ran a long screwdriver up and cleared the mud and installed a screen over it. Secondly if it will run only while you hold the start switch in then the brushes probably have built up carbon around them and on the communtator shaft or brushes are worn out, Remove the brushes/check and clean. Tape a piece of steel wool on a screwdriver and hold it against the commentator shaft while turning the engine over to clean it. DO NOT lose the steel wool down inside while doing this. Also the regulator has a drain plug on the bottom that needs to be removed to allow any oil/water that may have came from the propane to drain out. Manual requires this on a regular basis. I do it once annually with oil change

He explained that generator governor servo's circuits usually get their RPM signal input from the crankshaft sensor voltage. Even the most minute reduction in RPM causes the servo to open the carb butterfly a bit... then when the RPM exceeded specs carb closed a bit. At a point an over voltage circuit shut the generator down.

In my case there was no audible indication of an intermittent single plug misfire. But the rear cylinder plug was badly burned. The rear cylinder compression was 120 lbs. The front was 130.

Tech also suggested replacing air filter, checking exhaust tip to make sure no restrictions after long layup. Also consider de-carbon'ing the heads at 500+ hours.

I learned something a while ago about our propane generators. The propane regulators can be problematic for the oil in the propane plugging them up. Mine isn't quite so bad, I guess because it's a propane vapor system. Some however are propane liquid systems. On water cooled engines, there is a water coolant passage running through the regulator, so the regulator is always at engine operating temperature. Apparently this helps to make sure the oil in the propane stays liquid and remains easy to draw through the system and burned up by the engine. With our air cooled engines, there is no way to heat the regulator. To compensate for this, the regulator on my Onan is mounted right beside the muffler, in the hope that the heat from the muffler will heat up the regulator enough to keep the oils dissolved.

Here's the catch. If I don't run the engine for long enough periods of time, the regulator doesn't get a chance to get hot, so the oils slowly collect and solidify into a gelatin like substance that eventually plugs the regulator until the system just can't work properly.

So, I make sure to run that generator for about two hours each time I start it. It seems to be working. I had the regulator apart when I fixed the ignition module, and it was clean.